Last night, commuting, I got lots of dirty water all over my back brake (Promax Render-r mechanical disc), as well as on most of the rest of the bike, and me. I wiped the rotors with a clean rag to get the grit off, because light braking made an unpleasant grinding sound. This morning I was about 30km into a rather long ride when I realised I had essentially no back brake. I don't use the back much except to hold back on descents, though I do normally take up the slack when stopping, so it was the first decent descent when I discovered this.
I can only assume that the water was oily (certainly possible given the state of the roads and the deep puddles last night) and that I spread it evenly over the disc.
So I ended up riding 250km with essentially no back brake. I would have tried cleaning the rotor with an alcohol sterilising wipe from my first aid kit, but they'd dried up. I did get a bit off slowing (not stopping) power from holding the brake on all the way down long descents, and cooking it, but presumably this just moved the oil to the pads and finished them off.
I've ordered new pads (front and rear), which I needed soon anyway. I plan to degrease the rotor, put the used front pads in the back, and see what happens.
Now I've had the chance to remove the wheel and pads, the moving pad on the rear is unevenly worn -- much more wear towards the centre of the rotor. It was seated properly; I checked after takling the wheel off and before taking the pads out. I don't know whether this wear was cause or effect, as I had the back brake on hard on most downhills for the little effect it did have. There are some scratches on the back rotor but not much more than the front (or the MTB) and nothing deep like I'd expect if there was a bit of grit preventing proper contact. The pads were by no means clean but not gritty either.
But what could I have done at the side of the road? There were no bike shops around, but convenience stores and I could probably have found a pharmacy.